Fuel cell systems convert a fuel and an oxidant to electricity in a fuel cell stack. One type of fuel cell system employs a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of the fuel (such as hydrogen) and the oxidant (such as oxygen or air) to generate electricity. The PEM is a solid polymer electrolyte membrane that facilitates transfer of protons from an anode to a cathode in each individual fuel cell assembly of the fuel cell system. Electrodes, a catalyst, and the PEM are assembled to form a membrane electrode assembly (MEA).
In a typical PEM fuel cell assembly, the MEA is disposed between gas diffusion media (GDM). The GDM and MEA are disposed between a pair of electrically conductive plates. If the plates are bipolar plates, the plates conduct current between adjacent fuel cell assemblies in the fuel cell system. If the plates are unipolar plates at an end of a stack of fuel cell assemblies, the plates conduct current externally of the fuel cell assemblies.
Individual fuel cell assemblies include channels formed therein to facilitate a flow of the reactants and a cooling fluid therethrough. Fuel cell plates are typically designed with serpentine flow channels. Serpentine flow channels are desirable as they effectively distribute reactants over an active area of the fuel cell assembly, thereby maximizing performance and stability of the fuel cell assembly. Movement of water from the flow channels to outlet manifolds of the fuel cell plates is caused by the flow of the reactants through the fuel cell assembly. Water in PEM fuel cell systems may accumulate and form ice in subfreezing conditions. Repeated freezing and thawing of the PEM may reduce a useful life of the PEM. Additionally, a time required for a start-up operation of the fuel cell system is increased due to the presence of water and ice in the fuel cell system. A warm-up and drive away time of a vehicle including the fuel cell system is also increased.
Typically, a draining operation is used to remove the water in the manifolds of the fuel cell system during a shutdown operation. Water that remains in the fuel cell system after the draining operation may be removed from the fuel cell system with a shutdown purge. The shutdown purge may be a vacuum evaporation, an air purge, a cessation of the humidification of the reactants, or other similar fuel cell assembly humidity starvation methods known in the art.
To maintain high proton conductance and low internal resistance in the fuel cell system during a startup operation and normal operation, the PEM must maintain a desired level of hydration. Conventional shutdown purge procedures are typically intended to remove liquid from the flow channels of the fuel cell plates, GDM, electrode pores, and the PEM of the fuel cell system. Adequate removal of liquid water (from flow channels for example) often requires long purge durations, such that the process of moisture removal from the PEM results in an undesirable drying out of the PEM to a level below the desired level of hydration. A typical PEM will have a hydration index (λ) of approximately nine. The hydration index is defined as the number of moles of water per equivalent sulfonic acid group in the PEM. Following conventional shutdown purge operations, the PEM may have a hydration index below 3.5. If the hydration index of the PEM is less than nine, an Ohmic (voltage) loss in the fuel cell assembly will occur. Ohmic loss is defined as a voltage drop created by resistance to a flow of ions in the PEM and resistance to a flow of electrons through the electrode and the bipolar plate materials. During start-up operations in cold or freezing conditions, a hydration index less than nine may result in an increased Ohmic (voltage) loss in the fuel cell system, thereby further increasing the warm-up and drive away times.
It would be desirable to develop a method of operating a fuel cell system to optimize a start-up operation of the fuel cell system at temperatures which may cause a freezing of water in the fuel cell system.